For smashing, Lee Jae Bok recommends forehand grip turned slightly towards panhandle, to prevent slicing when hitting the shuttle in front of you.
BTW does the smash grip differ if you use the "pronation technique"? like this:http://www.youtube.com/watch?v=HNVC5PVJyPQ coach lee seems to use more of a flat slap http://www.youtube.com/watch?v=aIug4gRpDvg while the pronation technique guy turns the wrist to hit the ball. don't you need a "stronger" (golf term, I don't now how to explain but I mean less panhandle turning the other direction from the standard grip as lee suggests) grip if you turn the wrist in to square the ball up?
IMHO it's easier to change grip rather than change my arm swing mechanics for a smash. Having the same swing mechanics for all forehand strokes also provides deception: your wind up can easily be a smash, clear or drop...
If you lock your wrist in that position, this limits the power transfer you can impart to the shuttle, whether you use forearm pronation or not. If you look at slow motion videos of smashes you can see that although most of the swing speed is through pronation, a slight cocking and uncocking of the wrist adds a little bit extra to the racket swing speed.
With instructional videos, the motion is exaggerated to better illustrate the technique. So although LJB looks like he's hitting it with a panhandle grip, he stresses that it is only a small angle change from the forehand grip.
But, ultimately you can use whatever you're comfortable with. The best way to find out what works for you is to try.